1. Field of the Invention
The present invention relates to a magnetoresistive element of a ferromagnetic material.
2. Description of the Prior Art
Recently, magnetoresistive elements of ferromagnetic materials have been used for angular position detectors, magnetic sensors and the like. A magnetoresistive element of ferromagnetic material is an element in which the electric resistance varies depending upon the angle between the magnetization direction of the ferromagnetic material and the direction of an electric current passing therethrough when the ferromagnetic material is placed in a magnetic field.
Typical examples of magnetic materials having a magnetoresistance effect are Ni-Fe alloys, Ni-Co alloys and other ferromagnetic materials.
A typical example of such a magnetoresistive element of ferromagnetic material is shown in FIG. 1A and FIG. 1B, wherein FIG. 1A is a top plan view of a magnetoresistive element of ferromagnetic material and FIG. 1B is a sectional view of the element taken along the line A-A' of FIG. 1A. In the figures, there are shown a magnetoresistive element designated generally by the numeral 0. Element 0 has a magnetically sensitive member 1 formed in a pattern of narrow lines; wiring portions 2 formed so as to have widths of not less than several hundreds of microns; terminal portions 3 for external connection; and substrate 4. The sensitivity of such a magnetoresistive element is defined by .DELTA.R/R wherein .DELTA.R means magnitude of change in resistance of magnetically sensitive member 1 observed when a magnetic field is applied to the element, and R represents the resistance of the entire element. In order to enhance the sensitivity of the magnetoresistive element, it is required to make the value of .DELTA.R much higher and to reduce the percentage of the resistance of the wiring portion 2. For this purpose, a method in which, when the film material used for wiring portions 2 is the same as that of the magnetically sensitive member 1, it is conventional to make the film thickness of the wiring portions 2 equal to that of the magnetically sensitive member 1 while the width thereof is enlarged, as shown in FIGS. 1A and 1B for simplicity.
Such a method has the advantage that the pattern of an element can easily be formed. However, the method suffers from several inherent problems. First, it is difficult to miniaturize the element because the sheet resistance of thin films is large and, consequently, a large substrate area is necessary to reduce the electrical resistance of the wiring portions. Second, a desired sensitivity often cannot be obtained since it is impossible to sufficiently lower the resistance of the wiring portions (the resistance of the wiring portions being usually 5 to 10% of the entire resistance). Third, the resistance value of the wiring portions can become unbalanced, i.e., an offset voltage is generated at the output side of the magnetoresistive element even when a magnetic field is not applied thereto, which in turn exerts an adverse effect on the properties of the element. This conventional method also suffers from problems such that if a thin film of the same thickness and material as those of the magnetically sensitive member 1 is used to form wiring portions 2, the sheet resistance thereof becomes large. In such cases it is not possible to ensure sufficient reliability since the external electric connection is not attained without difficulty.
Another method has been proposed in which the film thickness of wiring portion 2 is thicker than that of magnetically sensitive member 1. Still another proposed method would use for the wiring portions 2, materials having a higher conductivity than that of the magnetically sensitive member 1. These methods are capable of miniaturizing the size of the magnetoresistive element. FIGS. 2A and 2B show such a conventional magnetoresistive element, wherein FIG. 2A is a top plan view and FIG. 2B is a sectional view taken along the line A-A' of FIG. 2A. In these figures, the reference numeral 22 represents a part of wiring portion 2 which is formed simultaneously with the magnetically sensitive member 1 from the same material as that of the latter, and 21 represents a wiring member formed on part 22. In this case, the sensitive member portions 1 of the magnetically sensitive element and the wiring portions 2 of the magnetoresistive element differ from one another in the film thickness. Such a construction provides advantages such that the wiring portion has a low sheet resistance.
However, since the magnetically sensitive members and the wiring portions are formed separately in this method, the alignment of the connections therebetween is difficult. Conventional elements generally have one or both of the outer edges of the magnetically sensitive member formed with a narrow width and at least one of the outer edge of the wiring portion arranged along a straight line. However, in such a construction, an offset voltage often occurs due to a slight discrepancy in alignment at the connections between the magnetically sensitive members and wiring portions, discrepancy is inevitably generated during the production of the magnetoresistive element.
FIG. 3A shows one example of a designed pattern for the connections between the magnetically sensitive members 1 and the wiring portions 2 wherein the film widths equal to one another. In practice, the magnetically sensitive members 1 and wiring portions 2 are not simultaneously formed and, therefore, a discrepancy in alignment can easily occur, as shown in FIG. 3B. As a result, breakage of wiring tends to occur at boundaries between magnetically sensitive member 1 and wiring portion 2 and thus the element produced is unreliable. Moreover, the offset voltage becomes large due to the discrepancy in alignment as discussed previously.
FIG. 4A shows one example of a designed pattern of connections between magnetically sensitive members 1 and wiring portions 2 which are widened at only one side thereof with respect to the magnetically sensitive member 1. A discrepancy in alignment likewise arises in this case as shown in FIG. 4B, and wiring breakage is apt to occur at the boundaries between the magnetically sensitive members and the wiring portions. In such a case the offset voltage becomes high and, accordingly, the element is not reliable. Thus, if the magnetoresistive element is designed so that all the magnetically sensitive members have the same value of resistance, the discrepancy in the resistance value between the magnetically sensitive members inevitably arises due to the discrepancy in the pattern for connections between the magnetically sensitive members 1 and the wiring portions 2 during the manufacture of the magnetoresistive element.
FIG. 5 is a top plan view of another magnetoresistive element produced in the same manner as that shown in FIGS. 2A and 2B. Although, this example differs in the arrangement of the magnetically sensitive members from that shown in FIGS. 2A and 2B, the shape of the connecting portions between the magnetically sensitive members and the wiring portions is identical with that of the conventional example shown in FIGS. 2A and 2B and, therefore, this example suffers from the same problems as those explained previously.
The offset voltage phenomenon of a magnetoresistive element will now be explained briefly. FIG. 6 is a diagram of the equivalent circuit of the magnetoresistive element shown in FIG. 2A. The offset voltage means a voltage appearing on the output terminal even when a magnetic field is not applied to the magnetoresistive element and the value thereof is expressed as the voltage difference between the designed voltage of the output terminal and the observed voltage. In the equivalent circuit shown in FIG. 6, the offset voltage Voff is defined as follows: Voff=V.sub.24 -V'.sub.24 on the hypothesis that the designed voltage difference is equal to V.sub.24 and the observed voltage difference V'.sub.24 between terminal electrodes 3-2 and 3-4, and is generated when an input voltage Vin is applied between the terminal electrodes 3-1 and 3-3.
In FIG. 6, R.sub.1, R.sub.2, R.sub.3 and R.sub.4 are resistance values of the wiring portions 2-1, 2-2, 2-3 and 2-4 shown in FIG. 2A respectively; and R.sub.12, R.sub.23, R.sub.34 and R.sub.14 are resistance values of the magnetically sensitive members 1 formed between the wiring portions 2-1 and 2-2, the wiring portions 2-2 and 2-3, the wiring portions 2-3 and 2-4 and the wiring portions 2-1 and 2-4 respectively. In addition, r.sub.14, r'.sub.14, r.sub.34, r'.sub.34, r.sub.23, r'.sub.23, r.sub.12 and r'.sub.12 represent combined resistance values of a part of the wiring portion 2 and the connecting portion respectively. In this connection, it is essential that R.sub.1, R.sub.2, R.sub.3, R.sub.4 and r.sub.14, r'.sub.14, r.sub.34, r'.sub.34, r.sub.23, r'.sub.23, r.sub.12 and r'.sub.12 are small in order to make the value of .DELTA.R/R large. Moreover, if the values of r.sub.14, r'.sub.14, r.sub.34, r'.sub.34, r.sub.23, r'.sub.23, r.sub.12 and r'.sub.12 differ from each other, a scatter occurs in the designed resistance values in a bridge circuit including magnetically sensitive members and thus an offset voltage is generated. In the case where the construction of the connecting portion shown in FIG. 3A or FIG. 4A is adopted, and if the alignment between the magnetically sensitive members 1 and the wiring portions 2 deviates only slightly from the desired one, a scatter is caused between the values of r.sub.12 to r.sub.14 and the values of r' .sub.12 to r'.sub.14, which leads to the generation of the offset voltage. This offset voltage degrades the properties of the element and a low yield in the production of the elements results.
As explained above, the conventional magnetoresistive elements and the attempts towards the improvement thereof can suffer from severe problems in that an offset voltage is generated due to a discrepancy in the pattern of connections between the magnetically sensitive members and the wiring portions, a large resistance value of the wiring portions, or the like.